Thermal Life of Varnished Glass Cloth

The suggested ASTM (American Institute for Testing Materials) and AIEE methods for the evaluation of the thermal stability of varnished glass fabric insulation are reviewed and the ASTM "Electric Breakdown Test Procedure" was found to be the better method, for it is more reproducible and gives a history of life of the varnish during thermal aging. The thermal stability of modified phenolic varnishes as measured by the ASTM breakdown procedure was substantiated by the thermal life evaluation of the same varnish in an insulation system by functional motor tests.